Characteristics of energy storage devices in piezoelectric energy harvesting systems

Using piezoelectric elements to harvest energy from ambient vibration has been of great interest recently. As the power harvested from the piezoelectric element is relatively low, energy storage devices are needed to accumulate the energy for intermittent use. In this study, the energy storage devices considered include rechargeable batteries and supercapacitors. The traditional electrolytic capacitors are not considered due to their low energy density. The charge/discharge efficiencies of the energy storage devices are of major concern. The equivalent circuit model of the energy storage devices is investigated. It is found that the leakage resistances of the energy storage devices are the dominant factor that influences the charge/discharge efficiency in the piezoelectric energy harvesting systems. A quick test method is proposed to experimentally study the charge/discharge efficiencies of the energy storage devices. The experimental results verify our findings. Adaptability, lifetime, and charging protection circuit of the energy storage devices are also discussed. It can be concluded that supercapacitors are suitable and more desirable than the rechargeable batteries to store the energy in the piezoelectric energy harvesting systems.